1. Field of the Invention
The present invention relates to a scanning optical equipment such as image forming equipment to scan a recording member such as photosensitive member by a flying spot coming from laser etc., particularly to a scanning optical equipment with stable spot diameter.
2. Related Background Art
With the conventional laser beam printer which employs electronic photosensitizer or silver salt photosensitizer, it has been customary that the state of formation of beam spot image on the photosensitive member is observed by guiding the beam to the photoelectric conversion device via a reflective mirror placed at a part on the line of laser scanning and based on the results of such observation, the state of formation of beam spot image on the photosensitive member is adjusted (for example, Japanese application Laid-Open No. 61-275868).
There was also a type where a reflective mirror is placed at outside the photosensitive member on the laser beam scanning line and thus the state of formation of beam spot image is observed in the same way (Japanese patent publication No. 60-9243).
However with such conventional equipments, the state of formation of beam spot image on the photosensitive member is observed not by directly detecting the beam from the photosensitive member but by what may be called an indirect mimetic method.
On the other hand, in recent years, with the rapid dissemination of desk top publishing, a higher picture quality is demanded to the laser beam recording. However according to the aforesaid conventional method, it has been known that there is certain limitation to the increase of resolution which is at around 400 dpi (dot per inch) to 600 dpi and to obtain the resolution of for example 1200 dpi or 2500 dpi responding to such demand for higher picture quality, other method of observation is believed to be necessary.
Now therefore, to solve the aforesaid problem, the objective of the present invention is to provide the scanning optical equipment which is so composed that the state of focussing of the spot on the irradiated member such as photosensitive member is directly detected.
In order to achieve the aforesaid objective, the scanning optical equipment of the present invention is provided with the spot detecting means which detects the state of focussing of the scanning beam on the scanned surface of the photosensitive member etc. by receiving the scanning beam having scanned the objective surface of photosensitive member etc. and the control means to control the state of focussing of the said scanning beam in response to the output signal emitted by the detecting means.
In the case of the aforesaid scanning optical equipment, the spot detecting means to observe the spot of the scanning beam to be focussed on the irradiated member such as photosensitive member is provided and therefore the state of focussing of the spot on the irradiated member is detected directly and with certainty and consequently such state of focussing can be precisely controlled.